Which one of the following is a X-linked recessive disease?

Question

Accepted Answer

The correct option is 1: Haemophilia

Haemophilia, particularly Haemophilia A and Haemophilia B, is the definitive and classical example of an X-linked recessive disorder. The disease is caused by a deficiency in one of the crucial blood clotting factors (Factor VIII for Haemophilia A and Factor IX for Haemophilia B), the genes for which are located on the X chromosome. Because males possess only one X chromosome (XY), a single copy of the recessive defective allele on that chromosome is sufficient to cause the disease, as there is no healthy dominant allele on a homologous chromosome to mask its effect. Females (XX) typically remain asymptomatic carriers, as the dominant normal allele on their second X chromosome compensates for the recessive mutated one. The characteristic inheritance pattern, where mothers pass the condition to their sons (crisscross inheritance), perfectly illustrates the principles of X-linked recessive transmission, making it the most accurate answer among the choices.

The incorrect option is 2: Diabetes insipidus

The term Diabetes insipidus (DI) is a broad medical term for a condition involving excessive thirst and urination, and most of its causes are not X-linked recessive. The most common forms, such as Central Diabetes Insipidus, are often acquired due to damage to the pituitary gland or are sometimes due to autosomal dominant/recessive gene mutations. While there is a specific and rare form called Nephrogenic Diabetes Insipidus (NDI) that *is* inherited in an X-linked recessive pattern, the general condition is too heterogeneous to be universally classified as such. Therefore, while a subset fits the description, it is not the classic, universally recognized example in the way Haemophilia is.

The incorrect option is 3: Pseudorickets

When discussing hereditary forms of rickets in the context of sex-linked inheritance, the most common type is X-linked Hypophosphatemic Rickets (XLHR). It is a disorder characterized by the kidney’s inability to properly retain phosphate, leading to bone deformities. Crucially, XLHR is inherited in an X-linked *dominant* pattern, not recessive. In this dominant mode, the affected allele is expressed even in the presence of a normal allele. This results in the disease affecting both males and females, which fundamentally contrasts with the X-linked *recessive* pattern that primarily affects males.

The incorrect option is 4: Hypertrichosis

Hypertrichosis refers to the condition of excessive hair growth over the body. Although there are indeed genetic forms of hypertrichosis linked to the X chromosome, the most widely documented congenital generalized forms, such as Congenital Generalized Hypertrichosis (CGH), follow an X-linked *dominant* pattern of inheritance. A dominant pattern means that only a single copy of the mutated gene is required for the disease to manifest. This is diametrically opposed to an X-linked *recessive* disease, where the disease allele is suppressed by a single normal allele in females and requires only one copy for expression in males.